1. Field of the Invention
The present invention relates to semiconductor laser elements of a strained quantum well type and a method for their production.
2. Description of Prior Art
A semiconductor laser element formed on a GaAs substrate with an active layer of a strained quantum well construction provided with an In.sub.x Ga.sub.1-x As (x=0.0 to 0.05) strained quantum well layer and a GaAs barrier layer is already known as a light source of wavelength of 0.9 to 1.1 .mu.m. This is a conventional lattice matched type laser, such as GaAs/AlGaAs and InAsP/InP.
In the case of semiconductor laser elements, as a clad for confining carrier and light in an active layer, a semiconductor should be used which has a permeability with respect to light having an oscillation waveform, is smaller in refractive index than that of the active layer (or a layer for confining light near the active layer), and is large in its energy gap.
In the conventional semiconductor laser element of an In.sub.x Ga.sub.1-x As strained quantum well type, Al.sub.w Ga.sub.1-w As of w&gt;0.2 is used as a clad.
FIG. 8 shows a conventional semiconductor laser element of an In.sub.x Ga.sub.1-x As strained quantum well type.
In FIG. 8, an n-type GaAs substrate 1 having approximately 350 .mu.m of thickness is formed thereon with an n-type GaAs buffer layer 2 having approximately 0.5 .mu.m of thickness and an n-type Al.sub.0.3 Ga.sub.0.7 As clad layer 3 having approximately 1.5 .mu.m thickness through epitaxial growth means such as the MBE or MOCVD method.
Further, in FIG. 8, the n-type GaAs substrate 1 is formed at its predetermined position with an essential portion 4 including an active layer provided with an In.sub.0.35 Ga.sub.0.65 As strained quantum well layer, a GaAs barrier layer, etc., and a light confining layer, a p-type Al.sub.0.3 Ga.sub.0.7 As clad layer 5 having 1.5 .mu.m of thickness and a p-type contact layer 6 having 0.2 .mu.m of thickness.
The details of part 4 are clearly shown in FIG. 9.
In FIG. 9, two upper and lower GaAs light confining layers 7 are 1500 .ANG. in thickness, a GaAs barrier layer 8 between these light confining layers 7 is 100 .ANG. in thickness and an In.sub.0.35 Ga.sub.0.65 strained quantum well layer 9 is 40 .ANG. in thickness.
A double hetero construction uniformly formed on the GaAs substrate 1 has a current restricting layer and an electrode, applied with microworking such as element separation, to prepare a laser chip.
One example of prior art has been described above. The semiconductor laser element has various modes such as a ratio of composition of mixed crystal, the number of layers of strained quantum wells, thickness of the layers, etc. As the light confining layer, the well known GRINSHC construction is used, including AlGaAs in which the ratio of the Al composition is parabolic.
Next, the process for working the bridge waveguide path into a strained quantum well type semiconductor laser element will be described hereinafter with reference to FIGS. 10(a) to 10(d).
In the process shown in FIG. 10(a), a resist 10 is patterned on a p-type GaAs contact layer 6 through means such as photolithgraphy.
In the process shown in FIG. 10(b), the resist 10 is used as a mask. The GaAs contact layer 6 of the double hetero construction and the upper clad layer 5 are subjected to etching until the depth of the etching reaches about 0.2 m of the active layer 4.
As etching liquids of the AlGaAs/GaAs type, there can be used a mixed solution of sulfuric acid and hydrogen peroxide, a mixed solution of tartaric acid and hydrogen peroxide, a mixed solution of ammonia and hydrogen peroxide or dry etching such as chlorine (for example, reactive ion beam etching).
In the process of FIG. 10(c), means such as spattering is used to form a surface of an epitaxial film with an etching mask 11 in the form of a film such as SiO.sub.2, SiN, etc.
In the process step of FIG. 10(d), means such as photolithgraphy is used to form an etching portion 12 in a stripe-like SiO.sub.2 as a patterned resist mask.
In the process steps that follow, electrodes are formed on both upper and lower surfaces of a laminate construction, and microworking, such as element separation, is applied thereto.
The technical tasks with regard to the aforementioned semiconductor laser element include oxidation of the Al, and providing compressive stress from a substrate lattice and defective etching process, which will be described hereinafter.
The main uses of laser having a 0.9-1.1 .mu.m waveform are excitation of a fiber amplifier in which a rare earth, such as Er is doped, or a visual light source in combination with SHG. In the cases of these semiconductor products, a prolonged service life is required at high output in excess of scores of mW.
However, the conventional strained quantum well type semiconductor laser cannot fulfill such a requirement as described above, since it uses the aforementioned AlGaAs as a clad layer. The reason is as follows.
Among elements (for example, In, Al, Ga, As, P, Sb, etc.) constituting a compound semiconductor, Al is an element which tends to be oxidized most easily. For example, when a regrowth surface is made by an embedding growth means during fabrication of a laser chip, oxidation of Al tends to occur.
Such an Al oxidation results in the occurrence of a non-light emitting center and the degradation of crystallization, and failing to obtain a semiconductor laser excellence in the laser characteristics.
Furthermore, in the case where a plaited or weave-link surface is used as a laser end, oxidation of the end progresses during use of laser to bring forth a lowering of refractive index, and an increase of absorption, deteriorating laser characteristic.
The semiconductor laser increases its temperature particularly during laser operation at high pouring, the progress of oxidation being sped up.
Means are proposed to remove oxygen and water content in the fabrication process of the laser, in order to suppress oxidation of Al. However, this requires much labor.
Because of this, in case of prior art, it is not possible to obtain a laser having a long service life under the use condition of high output.
In a double hetero construction for a laser diode using a conventional IN.sub.x Ga.sub.1-x As/GaAs strained quantum well construction as an active layer, semiconductors having a larger lattice constant than that of GaAs substrate are laminated.
That is, the lattice constant of the GaAs substrate is 5.65 .ANG. whereas Al.sub.w Ga.sub.1-w As has a large lattice constant, 0.14 w % and In.sub.x Ga.sub.1-x As has a large lattice constant, 7.3 x %.
Incidentally, assume that compositions of In.sub.x Ga.sub.1-x As and Al.sub.w Ga.sub.1-w As are x=0.35 and w=0.3, respectively, lattice non-matching rates with respect to GaAs are+2.65% and+0.04%, respectively.
In this case, the Al.sub.w Ga.sub.1-w As layer is small in the lattice non-matching rate with respect to GaAs but is thick, 3 .mu.m (about ten thousand atom layer). An In.sub.x Ga.sub.1-x As layer is thin, 120 .ANG. (about 40 atom layer), but the lattice non-matching rate with respect to GaAs is large and, therefore, a laminate of the Al.sub.w Ga.sub.1-w As layer and the In.sub.x Ga.sub.1-x As layer receives a compressive stress from the substrate. This compressive stress causes occurrence of transition and slip in the active layer of the strained quantum well construction during high pouring and the laser operation of high excitation.
As a result, the semiconductor laser element tends to give rise to DLD (dark line defect), lowering the laser oscillation life.